Alkyl-.alpha.-cyanoacrylate is one of the most commonly used "instant" adhesives. Conventionally, alkyl-.alpha.-cyanoacrylate is prepared using alkyl-.alpha.-cyanoacetate and formaldehyde as raw materials, which are reacted under an alkalinic environment to form poly(alkyl-.alpha.-cyanoacrylate). The poly(alkyl-.alpha.-cyanoacrylate) is then cracked (i.e., depolymerized) and purified to obtain high-purity alkyl-.alpha.-cyanoacrylate.
U.S. Pat. No. 2,756,251 discloses a method for the preparation of monomeric .alpha.-cyanoacrylate by which a polymeric ester of .alpha.-cyanoacrylic acid is heated in a tertiary ester of phosphoric acid, for example tricresyl phosphate, at a temperature above the melting point of the polymer to produce the monomeric .alpha.-cyanoacrylate. The amount of tricresyl phosphate required in the process disclosed in the '251 patent is about 0.9 times that of the polymeric raw material. The reaction is conducted as a batch process. Because of the high viscosity of the reaction mixture, very slow depolymerization rate is experienced. Typically, it takes between 5 and 10 hours to complete the reaction. The reaction time will be substantially prolonged if inadequate mixing is provided. When the reactant polymer is subject to long time exposure in the high-temperature (typically about 200.degree. C.) environment, it property will deteriorate causing the reaction yield to be adversely affected. Furthermore, large amounts of dark brownish residue will be produced after the depolymerization reaction. This causes significant difficulty in cleaning the reactor which also indirectly results in reduced production capacity.
U.S. Pat. No. 3,728,373 discloses a continuous method for making cyanoacrylic acid esters by depolymerization polycyanoacrylic acid esters. The polymer is first admixed with an inert liquid of high boiling point, such as tricresyl phosphate, and one or more polymerization inhibitors, such as phosphorus oxide and hydroquinone. Then the mixture is depolymerized by heating a thin layer of this mixture to produce high-purity monomeric .alpha.-cyanoacrylate gas. The monomeric .alpha.-cyanoacrylate gas is then condensed at -8.degree. C. to form the final product. The continuous process disclosed in the '373 patent is a substantially improved process over that disclosed in the '251 patent. However, it requires the use of large amounts of tricresyl phosphate (1.45 times the amount of the polycyanoacrylic acid esters), thus resulting in serious liquid pollution and disposal problems.